The problem that arises in this connection is that, in order that the clutch release bearing may be brought into engagement with the above mentioned thrust means of the clutch during assembly, the movable member of the actuator which carries the clutch release bearing must be in a position in which it is extended or projecting from the fixed member of the actuator; but, in order for the clutch release bearing to be in permanent engagement against the said thrust means in service through the coupling member, preloading means are also provided for, by contrast, biassing the movable member of the actuator permanently towards its retracted position with respect to the fixed member.
Accordingly, during assembly, the movable member of the hydraulic actuator first has to be put into its extended position and retained in that position, from which it subsequently has to be released. In other words, it is necessary to provide retaining means which are adapted to retain the movable member in its extended position for so long as this is required.
In the specification of French published patent application FR 2 651 846A, the preloading means comprise a spring of the coil spring type, and the retaining means comprise a retractable abutment element which, under control, provides an engagement means for the movable member of the hydraulic actuator at the rear of the latter. In order that this retractable abutment element can be effaced so that the movable member of the hydraulic actuator can be released from its extended position, a specific additional action on the retractable abutment element is necessary, and this complicates the assembly operations.
Such a retractable abutment element also increases quite substantially the radial size of the whole assembly, and it is also relatively complicated and costly to manufacture. In addition, because the abutment element gives the movable member of the hydraulic actuator a rigid mechanical abutment point, it is liable to give rise to damage of either the fixed or the movable member of the actuator when in use, if the axial thrust exerted in order to achieve the required snap-fitting action is too great.
Finally, since in practice there is no check as to whether the required snap-fitting engagement has indeed been achieved, it is possible that the retractable abutment element can be accidentally effaced, even when the snap-fitting has for some reason or other not been effected. Once this error becomes apparent, the operations have to be started all over again, and the movable member of the hydraulic actuator has to be restored to its extended position.
However, it can happen that this fault may just as easily pass unnoticed, so that the clutch is then subsequently unable to be disengaged.